California is home to eight of the 10 cities in America where air pollution is worst

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A combination of unfortunate topography, a large population, and the realities of worsening climate change makes California cities some of the worst places to breathe air in America.

“No state has done more to address air pollution than California has done over the last 50 years. But they also have the biggest problems,” says Paul Billings, the senior vice president of advocacy at the American Lung Association, which issued its latest State of the Air report on Tuesday (April 19).

Eight of the 10 cities with the highest year-round concentration of particulate matter—or PM2.5—between 2013 and 2015 were in California, according to the report. The state is also home to seven of the 10 American cities with the worst ozone pollution. Both PM2.5 and ozone are linked to an long list of health problems, including asthma, lung cancer, premature death, and developmental delays in children.

Most polluted metropolitan regions by average year-round concentration of particulate matter (PM2.5)

Visalia-Porterville-Hanford, CA

Bakersfield, CA

Fresno-Madera, CA

San Jose-San Francisco-Oakland, CA

Los Angeles-Long Beach, CA

Modesto-Merced, CA

El Centro, CA

Pittsburgh-New Castle-Weirton, PA-OH-WV

Cleveland-Akron-Canton, OH

San Luis Obispo-Paso Robles- Arroyo Grande, CA

Six of the top 10 cities with the biggest problem with unhealthy spikes in PM2.5 are in California, too. A “spike” is defined as a day when the concentration of PM2.5 rises above the limit set by the US Environmental Protection Agency for acceptably healthy air.

Bakersfield, California remains the most polluted city in America in terms of spikes in particulate matter, with Visalia, Fresno, and Modesto-Merced coming in second, third, and fourth. All four cities are in California’s Central Valley, where PM2.5 is made abundant by the high volume of oil and gas drilling and diesel engines in the area.

The Central Valley is also topographically cursed—it’s shaped like a bowl, so pollution is often trapped, unable to disperse, causing concentrations to rise to ever more unhealthy levels, according to Billings. Los Angeles, similarly, is stuck in a topographic bowl, which helped to land it at number one on a list of the most ozone-polluted cities in America (on that list, Bakersfield came in second).

Most polluted cities by ozone pollution

Los Angeles-Long Beach, CA

Bakersfield, CA

Fresno-Madera, CA

Visalia-Porterville-Hanford, CA

Phoenix-Mesa-Scottsdale, AZ

Modesto-Merced, CA

San Diego-Carlsbad, CA

Sacramento-Roseville, CA

New York-Newark, NY-NJ-CT-PA

Las Vegas-Henderson, NV-AZ

Both Los Angeles and the Central Valley cities are prone to “ozone inversions” due to that topography, where warmer air floats above cooler air, forming a sort of atmospheric lid that keeps the air stagnant and prevents pollution from exiting the area. Agriculture, too, can contribute to ozone: the prevalence of certain pesticides containing volatile organic compounds, or VOCs, a precursor to ozone formation, pose another problem for the agriculture-rich Central Valley.

Climate change is making all this worse—it means more severe periods of drought, and more hot, dry days mean more ozone formation. Climate change also creates prime conditions for more extreme wildfires, which produce both PM2.5 and the elements needed to form ozone.

“Climate change is making air pollution worse, and harder to clean up,” says Billings.

Despite all this glum news, air quality in America is improving on the whole. Although the American Lung Association found that 125 million people live in places with dangerous levels of air pollution, that’s still one-quarter fewer than in 2012-2014, the period analyzed by the Association’s prior report.

That tracks with the consistent improvement in US air quality documented since the EPA began enforcing the Clean Air Act in the 1970s. But, Billings says, that could start to reverse if the current presidential administration makes its proposed cuts to the agency—and to the local departments it supports.

“The cuts being proposed at the EPA will not only cripple the EPA’s enforcement of the Clean Air Act, it will also have a really negative impacts on local air pollution cleanup efforts—the president’s budget includes half a billion dollars in cuts to state and local grants. That’s money they use to clean the air,” Billings says. “Losing those dollars means that not only will the federal cop not be on the beat, it means that state and local agencies will be hamstrung as well. Which means we may lose some of the progress we saw in this report.”

“And when there’s lack of enforcement we also know cheating occurs — you only have to look at Volkswagen.”